This invention relates generally to a method and apparatus for cooling a motor, and in particular, to a novel closed-loop system for cooling a compressor having air bearings and an air-cooled motor.
Environmental control systems for the more electric aircraft (MEA) use a dedicated compressor to pressurize ambient air for use as fresh air in the cabin. The compressor may be, for example, a single stage centrifugal configuration driven by a variable speed permanent magnet motor. In traditional environmental control systems, the pressurized air for the cabin is bled from the main engine or the auxiliary power unit. The air can sometimes become contaminated with an odor due to the leakage of trace amounts of oil, for example, through shaft seals.
An essential attribute of the MEA environmental control system (ECS) is that the method of compressing ambient air be oil-free. To meet this requirement, the compressor rotating assembly is supported by air bearings and the motor is air-cooled. In order for the air bearings to operate properly, the motor/bearing cavity should be maintained at near cabin pressure. The cooling airflow rate required to maintain satisfactory stator winding temperatures is significant, about 0.3-0.4 lb/min per kW of motor power. At the aircraft maximum cruising altitude, the motor driven compressor can deliver 0.65 lb/min per kW of motor power. Furthermore, the low air density at high altitudes increases the need for large motors with the consequent payload increase. For this reason, the cabin air compressors are by far the largest users of electric power on the more electric aircraft. Given these parameters, it is not feasible to use all fresh air with an open-loop system for motor cooling due to the substantial equipment weight and fuel burn penalties.
An apparatus that attempts to use fresh air within an open-loop system to cool a motor is disclosed in U.S. Patent Application Publication No. 2004/0005228 A1 of Agrawal, et al. The publication describes an oil-free motor-driven compressor/blower for use in fuel cell systems. Fresh, ambient air may be used to cool the motor. The device disclosed in the publication is not suitable for use at high altitudes or aboard flight vehicles, as fresh, ambient air at these conditions is of low density. Using low-density air requires a larger compressor and excess power to achieve the same function as using higher density air for motor cooling. Furthermore, continuous compression of low-density fresh ambient air would be required to use the device of Agrawal et al. The device in the Agrawal publication is not suitable for use aboard an aircraft due to the constraints on equipment size, weight, and power usage of aircraft components.
As can be seen, there is a need for an improved apparatus and method for cooling an electric motor for driving an aircraft cabin air compressor without contaminating the cabin air with lubricants. Furthermore, there is a need for an apparatus and method for cooling an air-cooled motor that functions efficiently during flight.